The agency is charging Tui operator Australian Worldwide Exploration and the FPSO's owner-operator Prosafe Production. Each company faces a maximum fine of $NZ200,000 (about $A161,680) and could be ordered to meet any clean-up costs not already met.

Minority Tui partner New Zealand Oil & Gas said today the oil discharge was very disappointing given the partners' strong focus on environmental performance and the prosecution was expected.

AWE has already admitted that the source of the pollution was the accidental discharge of about 23,000 litres of oil-contaminated produced water from the Umuroa FPSO on October 21, which ended up as tar balls and contaminating sand along a 10km stretch of coastline about 60km from the Tui project a few days later.

Since then the joint venture and its contracted partners, in conjunction with the regulatory authorities, have introduced improved operational and management practices and procedures in an effort to prevent a recurrence of similar incidents.

AWE New Zealand manager Dennis Washer has already indicated his company is likely to make an early guilty plea and pay for the $NZ100,000 ($A80,840) clean-up carried out by the Taranaki Regional Council.

The Tui partners are operator AWE (42.5%), Mitsui E&P NZ (35%), NZOG (12.5%) and Pan Pacific Petroleum (10%).